1. Field of the Invention
The present invention relates to an organic electroluminescent display device and a production method thereof. More specifically, the present invention relates to an organic electroluminescent display device preferably used as an active matrix display device and a production method thereof.
2. Description of the Related Art
An organic electroluminescent (hereinafter, also referred to as an EL) display device includes an organic EL element in each pixel or dot. The organic EL element is composed of two opposed electrodes and an organic EL film interposed between these two opposed electrodes.
A passive matrix or active matrix system is commonly used as a driving system of the organic EL display device. Particularly in an organic EL display device with a large screen, an active matrix system is preferably used because a response speed is fast and a contrast ratio is excellent (for example, refer to Japanese Kokai Publication No. 2004-246320 and Japanese Kokai Publication No. 2001-35662). As shown in FIGS. 7 and 8, according to the active matrix organic EL display device, a gate line 111 and a source line 112 are arranged to be perpendicular to each other on a substrate 101 and a switching element such as a thin film transistor (hereinafter, also referred to as a TFT) and an organic EL element are arranged in each pixel or dot. A switching TFT 131 and a driving TFT 132 are generally arranged as the switching element, as shown in FIG. 7. An electrode 103 (hereinafter, also referred to as a lower electrode) on the substrate 101 side of two electrodes constituting the organic EL element is connected to the TFT, commonly. Further, a metal film is formed on the entire region to form an electrode 105 (hereinafter, also referred to as an upper electrode), as a common electrode, on the side opposite to the electrode 103 with the organic EL film 104 therebetween. In an example shown in FIG. 7, the lower electrode 103 is an anode and the upper electrode 105 is a cathode. Each TFT controls driving of the organic EL element. An interlayer insulating film 102 is generally formed between the substrate 101 and the lower electrode 103 except for a part needed for wirings. On the interlayer insulating film 102, a bank 106 is formed to surround the organic EL film 104. In FIG. 7, an electric current i flows from a power line 113 in the arrow direction under control of the driving TFT 132, and then flows to an external region 114 through an organic EL light-emitting region 107. In the driving TFT 132, G shows a gate electrode; S shows a source electrode; and D shows a drain electrode.
Such an organic EL display device is excellent in responsiveness and visibility such as contrast ratio and viewing angle. Further, the organic EL display device permits low power consumption, slim profile, light weight, and flexibilization of the display itself. In these points, the organic EL display device is very excellent. Therefore, future market expansion of such an organic EL display device has been expected as a next generation flat display. However, the technology of the organic EL display device is still developing in comparison to a liquid crystal display device or a plasma display device, which is a major display device at present. The organic EL display device still has a room for improvement in image qualities particularly when the device displays an image on a large screen.
By the way, a silicon thin film formed of amorphous silicon and polysilicon may be mentioned as a semiconductor material for a common TFT used in the organic EL display device. The polysilicon has a high field-effect mobility, but it is difficult to form a polysilicon film over a large screen panel in terms of production processes. Therefore, it is possible that amorphous silicon which can be easily formed over a large screen is used to form the panel. However, with regard to the amorphous silicon, only a method of forming an n-channel TFT has been established because of the production processes.
As shown in FIGS. 7 and 8, usually, the lower electrode (anode) 103 of the organic EL element is connected to the source electrode 134 of the driving TFT 132, thereby connecting the organic EL element to the TFT. According to such a connection method, if the resistance of the organic EL film 104 is changed with time, the gate voltage of the driving TFT 132 is changed. Therefore, it might become difficult to drive the organic EL element at a constant electric current. For this problem, a method in which the organic EL element structure is inverted, that is, the organic EL element has a bottom cathode structure in which the cathode is positioned at the bottom (on the substrate side), and then the cathode is connected to an n-channel TFT has been proposed (for example, refer to Jan Blochwitz-Nimoth et al., “Full Color Active Matrix OLED Displays with High Aperture Ratio”, “SID Digest of Technical Papers”, (U.S.), Society for Information Display, 2004, p. 1000, and ““Ekisho ni ha makenai” yuki EL ga kudougijyutu de teikosutoka he”, “Nikkei Micro-device, February issue, 2005”, Nikkei BP Corp., p. 77). However, if a polymer is used as a material for the organic EL, a material with a relatively low work function is used as a material for the cathode. Therefore, the cathode material is deteriorated during processes of forming the organic EL film, which inevitably leads to deterioration of the organic EL element. In this point, such a method has room for improvement.